Series/parallel connection scheme for energy storage devices

ABSTRACT

A battery pack has series-parallel connected battery cells. An inrush current limiting circuit has current limiting devices and a current limiting tracking means limits inrush current from shorted battery cells. Each current limiting tracking means is coupled to the current limiting devices such that they limit an inrush current from parallel neighbor battery cells when one battery cell on the column shorts. The current limiting devices are variable buffer resistors with a positive temperature coefficient. The current limiting tracking means are heat sinks that thermally couples the buffer resistors together such that the buffer resistors increase in resistance with a temperature increase caused by current flowing through the buffer resistor associated with the shorted battery cell. Current regulating elements are in series with columns of battery cells for preventing inrush current to the columns of the battery cells.

RELATED PATENT APPLICATIONS

U.S. patent application Ser. No. 12/459,654, filed on Jul. 6, 2009assigned to the same assignee as the present invention, and incorporatedherein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to batteries, in particular to large capacitysecondary batteries comprising multiple cells arranged in aseries-parallel structure. More particular this invention relates tomultiple cell batteries having current limiting devices to protect thebatteries during a cell shorting event.

2. Description of Related Art

In order to achieve desired voltage and capacity, a battery oftenconsists of many cells. A conventional series-parallel scheme ischaracterized by connecting a string of cells in series together, thenconnecting multiple of strings in parallel. A conventionalparallel-series scheme is characterized by connecting a bank of cells inparallel, then connecting a plurality of banks in series.

As known in elementary physics, cells (or strings of cells) connected inparallel tend to achieve and maintain identical voltages. Thespontaneous voltage equalization within a bank simplifies voltagemonitor and control processes during normal usage. But it is dangerousin case one of the cells (or strings of cells) develops an internalshort. For brevity the cell (or string of cells) containing the shortwill be called a shorted cell (or shorted string of cells). Other cells(or strings of cells) in the bank or row will be called parallelneighbors. The shorted cell (or string of cells) receives energy fromits parallel neighbors, through a spontaneous inrush current, and becomemore likely to overheat.

In a parallel-series battery structure, multiple banks are connected inseries. Inrush current is primarily circling around the bank harboringthe short. In a series-parallel scheme, there is only one bank ofstrings. Inrush current loops around the entire battery. Loop resistanceis greater in a series-parallel battery structure than in aparallel-series structure. Thus the inrush current is smaller in aseries-parallel structure than in a parallel-series structure. As aresult, a shorted cell is less likely to overheat in a series-parallelbattery structure than in a parallel-series battery structure. In otherwords, a series-parallel structure is more robust against internalshort, thus safer than a parallel-series structure.

However in normal usage, each string of the series-parallel batterystructure needs to be monitored and balanced independently, whereas theparallel-series battery structure can be monitored and balanced as asingle string. Thus a series-parallel structure requires more batterymanagement units (BMU) than a parallel-series structure. This economicalconsideration has lead to popularity of parallel-series scheme overseries-parallel scheme, at the expense of safety.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a matrix ofseries-parallel connected battery cells with intra-row isolation suchthat inrush current is reduced when a cell or string of cellsexperiences an internal short.

To accomplish at least this objective, a battery pack has a matrix ofseries-parallel battery cells. The battery pack has an inrush currentlimiting circuit to prevent current from a shorted battery cell fromdamaging other battery cells. The series-parallel connected batterycells or battery cells strings are arranged in rows and columns suchthat in each column, the battery cells are series connected such thateach battery cell in the column has a positive node connected to form ajunction with a negative node of an adjacent battery cell. A positivenode of a first battery cell of each column of battery cells isconnected to a positive output connector of the battery pack and anegative node of a last battery cell of each column of battery cells isconnected to a negative connector of the battery pack.

Each of the buffer resistors has a first terminal coupled to onejunction of the positive node of one of the battery cell or string ofcells in a column with the negative node of the adjacent battery of thecolumn. A second terminal of the buffer resistors connected to thejunctions a row of the battery cell or string of cells is connectedtogether and to a measurement node connector of a battery managementsystem. Each of the buffer resistors are variable resistors that willincrease resistance when one or more battery cell or string of cellsshort to prevent excess current from damaging other batteries in thebattery pack.

In some embodiments, each of the buffer resistors has a positivetemperature coefficient such that as a temperature increases within thebuffer resistor the resistance of the buffer resistor increases. Inother embodiments, each of the buffer resistors is a fuse that destructswhen one battery cell or string fails.

In various embodiments, the buffer resistors connected to the batterycells or string of cells of each column are thermally coupled such thatthe resistance of the buffer resistors is varied synchronously. Invarious embodiments, the buffer resistors are joined to a heat sink andhave an electrical insulator providing electrical isolation of thebuffer resistors from the heat sink. The heat sink is copper, aluminum,tin, lead or other heat conducting material. To insure uniform heatingof the buffer resistors, the buffer resistors are surrounded or nearlysurrounded by the heat sink to thermally isolate the buffer resistorsfrom the ambient temperature. The electrical insulator similarlysurrounding the buffer resistors to insure electrical isolation betweenthe buffer resistors and the heat sink.

In some embodiments, each column of the battery cells have a currentregulating element placed in series with the string of the battery cellsto prevent overcharging or over-discharging of the battery cells in theevent of a shorted battery cell. In various embodiments, the currentregulating element and the buffer resistors for each cell of this columnare thermally coupled to equalize the resistance of the bufferresistors. In various embodiments, the current regulating element is aresistor with a positive temperature coefficient to spontaneouslyequalize the current to suppress inrush current in each of the rows ofthe battery cells.

In various embodiments, the buffer resistors and the current regulatingelement are thermally isolated from the ambient environment. In someembodiments, an assembly of thermally coupled buffer resistors and thecurrent regulating element are surrounded with a thermal insulator tothermally isolate the buffer resistors and current regulating elementfrom the ambient temperature.

In other embodiments an inrush current regulating assembly has multiplepositive temperature coefficient buffer resistors coupled to thejunctions of a row of series connected battery cells in a matrix ofseries-parallel connected battery cells. The positive temperaturecoefficient buffer resistors are thermally coupled and electricallyisolated. In some embodiments, the positive temperature coefficientbuffer resistors are affixed to a heat sink with an electrical insulatorplaced between the positive temperature coefficient buffer resistors andthe heat sink. The heat sink is copper, aluminum, tin, lead or otherheat conducting material. To insure uniform heating of the bufferresistors, the buffer resistors are surrounded or nearly surrounded bythe heat sink to thermally isolate the buffer resistors from the ambienttemperature. The electrical insulator similarly surrounding the bufferresistors to insure electrical isolation between the buffer resistorsand the heat sink.

In some embodiments, the inrush current regulating assembly has acurrent regulating element in series with each column of the batterycells. The current regulating element is, in various embodiments, apositive temperature coefficient buffer resistor. The current regulatingelement is thermally coupled to and electrically isolated from thebuffer resistors of the column of the battery cells that is series withthe current regulating element. In various embodiments, the currentregulating element is a resistor with a positive temperature coefficientto spontaneously equalize the current to suppress inrush current in eachof the rows of the battery cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a series-parallel battery pack withmeasurement nodes shared by multiple columns of series of batteriesembodying the principles of the present invention.

FIGS. 2 a and 2 b are drawings of a side view and a top view of anembodiment of an assembly of buffer resistors embodying the principlesof the present invention.

FIG. 3 is a drawing of a side view of another embodiment of an assemblyof buffer resistors embodying the principles of the present invention.

FIG. 4 is a schematic diagram of an embodiment of a battery pack coupledto the measurement nodes of a battery management unit (BMU), where thebattery pack has a current regulating unit embodying the principles ofthe present invention.

FIG. 5 is a drawing of a side view of another embodiment of an assemblyof buffer resistors and a current regulating element embodying theprinciples of the present invention.

FIG. 6 is a plot of resistance of positive thermal coefficientresistance devices embodying the principles of the present inventionversus temperature.

FIG. 7 is a drawing of a top view of another embodiment of an assemblyof buffer resistors embodying the principles of the present invention.

FIG. 8 is a drawing of a side view of another embodiment of an assemblyof buffer resistors embodying the principles of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

U.S. Patent Publication 2011/0003182 A1 (Zhu) describes a hybrid batterypack that combines the safety of series-parallel and economy ofparallel-series schemes. The structure is a modified series-parallelscheme, in which any connection between two serially connected cells iscalled a junction. Each junction on the same row, i.e., at the samenominal potential, is coupled through a buffer resistor to a commonnode. A battery management unit (BMU) monitors electrical potential ofeach node, and potentials of the battery terminals. Alternately, thestructure may also be viewed as a modified parallel-series scheme, inwhich parallel connections within each bank of cells have non-negligibleresistance.

In a conventional series-parallel scheme, voltage of cells on the samerow tends to diverge, at a time-constant in the order of months. In aconventional parallel-series scheme, the time-constant of harmful inrushcurrent during internal short is in the order of seconds. In thestructure described in Zhu, the resistance of buffer resistors isselected so that the time-constant of spontaneous intra-bank voltageequalization is intermediate of two time-constants mentioned above. Thusthe hazard of inrush current is reduced and the need for independentmonitor and balancing for each string of battery cells is eliminated.

In addition to internal short and intra-bank equalization, optimizationof the buffer resistance is subject to additional considerations. Forexample, cells in a same string may possess different capacities. Alower buffer resistance allows more operating (charge/discharge) currentto bypass a cell of lower capacity, thus avoiding overloading andpremature degradation of this cell.

In another consideration, inrush current in a series-parallel scheme mayovercharge good cells on a shorted string, and over-discharge cells onparallel neighbor strings of battery cells. Similar damages may occur inthe structure describe in Zhu, if the buffer resistance is too large. Asan illustrative example, when a battery cell is shorted and its bufferresistor is too large, the battery cells resident on the parallelneighbor columns together may overcharge the remaining cells on thecolumn of battery cell having the shorted battery cell. The batterycells on the parallel neighbor columns not having the shorted battercells may over-discharge themselves. Given these additionalconsiderations, the buffer resistance becomes more difficult tooptimize.

To compensate for the overcharging and over-discharging of the batterycells, a current limiting device replaces the buffer resistors of Zhu tolimit the inrush current during a battery cell shorting event. Invarious embodiments, the current limiting devices are variable bufferresistors that increase their resistance when the current through themexceeds a certain amount. In some embodiments, the variable bufferresistors have a positive temperature coefficient.

FIG. 1 is a schematic diagram of a series-parallel battery pack 100 withmeasurement nodes MN₁, MN₂, and MN₃ shared by multiple columns of seriesconnected batteries B₁₁, B₁₂, . . . , B₃₃, B₃₄ embodying the principlesof the present invention. The batteries B₁₁, B₁₂, . . . , B₃₃, B₃₄ arearranged in rows and columns where the batteries B₁₁, B₁₂, . . . , B₃₃,B₄₄ of each column are connected in series with the positive terminalsof first batteries of each series B₁₄, B₂₄, and B₃₄ being connected to apositive terminal T⁺ of the battery pack 100. The negative terminal of alast batteries of each series B₁₁, B₂₁, and B₃, being connected to thenegative terminal T⁻ of the battery pack 100. The batteries B₁₁, B₁₂, .. . , B₃₂, B₃₃ resident on each column have their positive terminalsconnected to the negative terminal of the adjacent battery B₁₂, B₁₃, . .. , B₃₃, B₃₄. It should be noted that although FIG. 1 shows threecolumns of four serial connected batteries B₁₁, B₁₂, . . . , B₃₃, B₃₄, amore general configuration has any number m of columns of any number nof serially connected batteries B₁₁, B₁₂, . . . , B_(m(n-1)), B_(mn), iswithin the scope and intent of the present invention. Further thebatteries B₁₁, B₁₂, . . . , B₃₃, B₃₄ are shown as single cells, howeverhaving multiple cells serially connected as a string, or connected inparallel as a bank, or connected by a combination of series and parallelconnections as an array embodies the principles of the presentinvention.

The battery pack 100 has multiple current limiting devices that areimplemented as variable buffer resistors R₁₁, R₁₂, . . . , R₃₂, R₃₃.Each of the variable buffer resistors R₁₁, R₁₂, . . . , R₃₂, R₃₃ has afirst terminal coupled to one junction of the positive node of one ofthe batteries B₁₁, B₁₂, . . . , B₃₂, B₃₃ in a column with the negativenode of the adjacent battery B₁₂, B₁₃, . . . , B₃₃, B₃₄ of the column.The second terminals of the variable buffer resistors R₁₁, R₁₂, . . . ,R₃₂, R₃₃ that are connected to the junctions a row of batteries B₁₁,B₁₂, . . . , B₃₂, B₃₃ are connected together and to the measurementnodes MN₁, MN₂, and MN₃. The measurement nodes MN₁, MN₂, and MN₃ are theinput terminals for of a battery management unit BMU. The batterymanagement unit BMU monitors and balances voltage between successivemeasurement nodes, MN₁, MN₂, MN₃, and the positive terminal T⁺ and thenegative terminal T⁻ of the battery pack 100 (i.e., the voltage of eachrow of the batteries B₁₁, B₁₂, . . . , B₃₃, B₃₄). It is known in theprior art that a battery management unit BMU typically contains amultiplexer (not shown), which allows multiple rows to be monitoredsuccessively, using a single analog to digital (ND) converter. Thebattery management unit BMU may also establish a shunt path to drainrows of the batteries B₁₁, B₁₂, . . . , B₃₃, B₃₄ of excess charge, ortransfer the excess charge to rows of the batteries B₁₁, B₁₂, . . . ,B₃₃, B₃₄ that are deficient in charge.

Each of the variable buffer resistors R₁₁, R₁₂, . . . , R₃₂, R₃₃ willincrease resistance when one or more battery cell or string of cellsshort to prevent excess current from damaging other batteries in thebattery pack.

In various embodiments, the variable buffer resistors R₁₁, R₁₂, . . . ,R₃₂, R₃₃ are positive temperature-coefficient (PTC) devices. Thevariable buffer resistors R₁₁, R₁₂, . . . , R₃₂, R₃₃ have a lowresistance under normal operating and storage conditions, but theresistance increases automatically when a serious internal short occurs.The excessive inrush current heats the positive temperature coefficientvariable buffer resistors R₁₁, R₁₂, . . . , R₃₂, R₃₃ whose resistanceincreases with the heat transferred directly from the shorted cells ordue to the heat generated as result of the inrush current. The positivetemperature coefficient resistors are known in the art and are used as astandard safety device in cylindrical lithium-ion energy cells. Thepositive temperature coefficient variable buffer resistors R₁₁, R₁₂, . .. , R₃₂, R₃₃ do not carry normal operating current, thus neitherdissipating energy nor generating heat. The positive temperaturecoefficient variable buffer resistors R₁₁, R₁₂, . . . , R₃₂, R₃₃ differfrom that used in conventional cells. The positive temperaturecoefficient resistors of the cylindrical lithium-ion energy cells of theprior art are annular (resembling a gasket). The positive temperaturecoefficient variable buffer resistors R₁₁, R₁₂, . . . , R₃₂, R₃₃ are, invarious embodiments, a strip of positive temperature coefficientresistive material similar to that in the automobile rearwindow-defroster. The positive temperature coefficient resistivematerial may be made of barium titanate ceramics, or graphite andcristobalite-type SiO₂ composite.

In other embodiments, the variable buffer resistors R₁₁, R₁₂, . . . ,R₃₂, R₃₃ are fuses. The resistance of the fuses is negligible undernormal circumstances, and becomes infinitely large after a seriousinternal short. In various embodiments, the fuse is able to be reset. Aresettable fuse is commonly referred to as a thermal cut off unit and ismore expensive. The thermal cut off unit, however, reduces the effort ofservicing blown fuses.

In various embodiments, fixed buffer resistors R₁₁, R₁₂, . . . , R₃₂,R₃₃ as described in Zhu, positive temperature coefficient variablebuffer resistors R₁₁, R₁₂, . . . , R₃₂, R₃₃, and fuses are coupled in avariety of series and/or parallel combinations, to achieve desiredvariable resistance under diverse conditions. In general, the object isto achieve low buffer resistance during normal operations, to emulate aparallel-series is battery pack structure for the ease of managementwhile achieving high buffer resistance when an internal short occurs toachieve a series-parallel battery pack structure for safety.

In the above described embodiments, sporadic increases of the resistanceof the any of the variable buffer resistors R₁₁, R₁₂, . . . , R₃₂, R₃₃may cause adverse side effects. For example, if the buffer resistors R₁₃associated with the shorted battery (or battery cells) B₁₄ hasdestructively formed an open circuit, while all the other bufferresistors R₁₁, R₁₂, . . . , R₃₂, R₃₃ remain at a very low resistance.Batteries (or battery cells) B₂₃ in series with B₂₄, and B₃₃ in serieswith B₃₄ will overcharge battery cell B₁₃ and over-discharge themselves.This is more serious than in a conventional series-parallel scheme,because the voltage ratio of overcharge is 2:1 instead of 4:3. Invarious embodiments, this problem is eliminated by all the variablebuffer resistors R₁₁, R₁₂, . . . , R₃₂, R₃₃ on the same string or columnare made to change synchronously. For example, the buffer resistors R₁₁,R₂₁, R₃₁, buffer resistors R₁₂, R₂₂, R₃₂, and buffer resistor R₁₃, R₂₃,R₃₃ are on the same string and are thermally coupled such that theirtemperatures rise and fall together. This causes their resistances toall change in unison. This coupling of the buffer resistors R₁₁, R₁₂, .. . , R₃₂, R₃₃ that are on the same string effectively preventsovercharging and/or over-discharging during internal short, especiallyif the battery consists of long strings of battery cells on each stringof batteries (or battery cells) B₁₁, B₁₂, . . . , B₃₃, B₃₄.

FIGS. 2 a and 2 b are drawings of a side view and a top view of anembodiment of an assembly for a thermally coupled string of bufferresistors R₁₁, R₁₂, . . . , R₃₂, R₃₃. The buffer resistors R₁, R₂, andR₃ on the same string are disposed upon a common heat sink HS. Thebuffer resistors R₁, R₂, and R₃ are separated from the heat sink HS byan electrical insulator ES. For example, a thin film of plastic, such aspolyimide, may be coated on the heat sink to form desired electricalinsulator ES. Then multiple strips of PTC may be laid upon the ES toform desired buffer resistors. The plastic film may be reinforced withfiberglass or alumina powder to improve heat endurance.

In various embodiments, heat sink HS is a metal such as copper,aluminum, lead, tin or any other suitable heat conductor that isadequate to maintain substantially uniform temperature among the bufferresistors R₁, R₂, and R₃ on the same string. The thermal capacity of theheat sink HS is designed such that buffer resistors R₁, R₂, and R₃associated with a shorted cell will heat up the heat sink, hence allbuffer resistors on the heat sink, within a predetermined delay. Thebuffer resistors R₁, R₂, and R₃ associated with each column or string ofbatteries B₁₁, B₁₂, . . . , B₃₃, B₄₄ has its own heat sink HS.

FIG. 3 is a drawing of a side view of another embodiment of an assemblyof buffer resistors R₁, R₂, and R₃ that ensures uniform temperature riseof the buffer resistors R₁, R₂, and R₃ within a string of the bufferresistors R₁, R₂, and R₃ associated with a shorted battery B₁₁, B₁₂, . .. , B₃₃, B₃₄. In various embodiments, the buffer resistors R₁, R₂, andR₃ are placed on a first heat sink HS1 with a first electrical insulatorE11 adhered to the first heat sink HS1. The buffer resistors R₁, R₂, andR₃ are formed on the first electrical insulator EI1. A second electricalinsulator E12 is adhered to a second heat sink HS2. The second heat sinkHS2 with the disposed second electrical insulator E12 is placed on thefirst heat sink HS1 such that the buffer resistors R₁, R₂, and R₃ areenclosed to insure more complete thermal coupling of the bufferresistors R₁, R₂, and R₃. The thermal coupling of the buffer resistorsR₁, R₂, and R₃ causes their temperatures rise and fall together andcause their resistances to all change in unison. This coupling of thebuffer resistors R₁, R₂, and R₃ that are connected to the same column orstring of batteries B₁₁, B₁₂, . . . , B₃₃, B₃₄ effectively preventsovercharging and/or over-discharging during internal short. However, theinrush current can be large enough to be of concern, even if all bufferresistors R₁, R₂, and R₃ are effectively open. This is true especiallyif the battery pack 100 of FIG. 1 consists of short column or string ofbatteries B₁₁, B₁₂, . . . , B₃₃, B₄₄. The inrush current may passthrough the largest loop, i.e., through the positive terminal T+ and thenegative terminal T− of the battery pack 100. In this case, it isdesirable that a current regulating element be placed in series witheach column or string of batteries B₁₁, B₁₂, . . . , B₃₃, B₃₄.

FIG. 4 is a schematic diagram of an embodiment of a battery pack 100coupled to the measurement nodes of a battery management unit BMU, wherethe battery pack 200 has current regulating units CR₁, CR₂, CR₃embodying the principles of the present invention. Each column of thebatteries B₁₁, B₁₂, . . . , B₃₃, B₃₄ has a current regulating unit CR₁,CR₂, CR₃ in series with column or string of batteries B₁₁, B₁₂, . . . ,B₃₃, B₃₄ on the column. Although current regulating units CR₁, CR₂, CR₃,as shown, are connected between the negative terminal T− of the batterypack 200 and rows of the batteries or strings of battery cells B₁₁, B₁₂,. . . , B₃₃, B₃₄, it should be noted that the current regulating unitsCR₁, CR₂, CR₃ may be inserted anywhere in the series of the batteries orstrings of battery cells B₁₁, B₁₂, . . . , B₃₃, B₃₄. In variousembodiments, the current regulating units CR₁, CR₂, CR₃ are variableresistors, which may be adjusted manually during assembly andmaintenance, or controlled automatically during application. The currentregulating unit CR₁, CR₂, CR₃ are, in other embodiments, constructed ofone or more of the following elements:

-   -   1. a switch, relay, or contact, which is inexpensive, yet useful        in diagnostics, and allows a battery pack to work at reduced        capacity, in case one series fails.    -   2. a fuse that can be activated by either temperature or        current, where the fuse may or may not be able to be reset. A        fuse protects the column or string of batteries B₁₁, B₁₂, . . .        , B₃₃, B₃₄ from overload, or operating at excessively high        temperature.    -   3. a positive thermal coefficient resistance device in which        resistance increases monotonically, reversibly, and nonlinearly        with temperature. A positive thermal coefficient resistance        device may offset the negative thermal coefficient of cell        internal resistance, thus reducing the sensitivity of current        distribution to thermal gradient within the battery pack. A        positive thermal coefficient resistance device also functions as        a thermal fuse that can be restored.    -   4. a low-resistance adjustable resistor, for example a strip of        conductor whose resistance may be decreased by adding a        conductor in parallel (e.g., adding solder on the surface of        strip, or soldering additional conductor to the strip), and        increased by blocking the conductive path (e.g., punching a hole        on the strip, or cutting off a portion of the strip). It is        useful during assembly and maintenance.    -   5. a current regulator circuit.    -   6. an electronic switch (e.g. a CMOS device), which may be used        to control current distribution by feedback control. Any design        and construction of the current regulating units CR₁, CR₂, CR₃        that performs the function of preventing a large inrush current        from each of the column or string of batteries B₁₁, B₁₂, . . . ,        B₃₃, B₃₄ is in keeping with the principles of the present        invention.

The current regulating unit CR₁, CR₂, CR₃ must be extremely reliable. Asingle failure may open a series column string of the batteries orbattery cells B₁₁, B₁₂, . . . , B₃₃, B₃₄ and divert its share of currentonto other series column strings of the batteries or battery cells B₁₁,B₁₂, . . . , B₃₃, B₃₄ thus overloading them. Current regulating unitsCR₁, CR₂, CR₃ must also exhibit low power loss and heat generation.Efforts should be made to reduce or eliminate the need for unreliable orheat generating components.

FIG. 5 is a drawing of a cross-section of another embodiment of anassembly of buffer resistors R₁, R₂, and R₃ and a current regulatingunit CR embodying the principles of the present invention. The bufferresistors R₁, R₂, and R₃ are disposed on an electrical insulator EI thatis adhered to the current regulating unit CR. In various embodiments thecurrent regulating unit CR is a positive thermal coefficient resistancedevice.

FIG. 6 is a plot of resistance of positive thermal coefficientresistance devices embodying the principles of the present inventionversus temperature. Under normal operating conditions, the heatgenerated by charging/discharging current through the current regulatingunit CR and inter-string spontaneously equalizing current areinsufficient to raise the temperature of the assembly of bufferresistors R₁, R₂, and R₃ and a current regulating unit CR beyond thetransition temperature of current regulating unit CR. Therefore powerdissipation is low, and intra-bank equalization is efficient. Howeverwhen an internal short creates excessive inrush current, the positivethermal coefficient resistance device of the current regulating unit CRand of the buffer resistors R₁, R₂, and R₃ together generate enoughheat, to raise the temperature above transition temperature. Inrushcurrent of the assembly of buffer resistors R₁₁, R₂₁, R₃₁, and acurrent, regulating unit CR is effectively suppressed in all pathspreventing short-induced overcharging and over-discharging, especiallyif the battery pack 200 matrix of batteries or strings of battery cellsB₁₁, B₁₂, . . . , B₃₃, B₃₄ consists of short strings, i.e., series offew cells.

FIG. 7 is a drawing of a cross-section of another embodiment of anassembly of buffer resistors R₁₁, R₂₁, R₃₁ and a current regulating unitCR to ensure uniform temperature rise of the buffer resistors R₁, R₂,and R₃ and the current regulating unit CR within a column or string ofbatteries B₁₁, B₁₂, . . . , B₃₃, B₃₄ associated with a shorted battery.In various embodiments, the buffer resistors R₁, R₂, and R₃ are disposedon a first electrical insulator EI1 that is adhered to the first heatsink HS1. An opening is formed in the first heat sink HS1 to accept thecurrent regulating unit CR. The current regulating unit CR is secured tothe first heat sink HS1 with an electrical insulator E13 thatelectrically isolates the current regulating unit CR from the heat sinkHS.

A second electrical insulator E12 is adhered to a second heat sink HS2.The second heat sink HS2 with the disposed second electrical insulatorE12 is placed on the first heat sink HS1 such that the buffer resistorsR₁, R₂, and R₃ are enclosed to insure more complete thermal coupling ofthe buffer resistors R₁, R₂, and R₃ and the current regulating unit CR.The thermal coupling of the buffer resistors R₁, R₂, and R₃ and thecurrent regulating unit CR causes their temperatures to rise and falltogether and cause their resistances to all change in unison. Thiscoupling of the buffer resistors R₁, R₂, and R₃ and the currentregulating unit CR that are on the same string effectively preventsovercharging and/or over-discharging during internal short.

FIG. 8 is a drawing of a cross-section of another embodiment of anassembly of buffer resistors R₁, R₂, R₃, and R₄ and a current regulatingunit CR embodying the principles of the present invention. To furtherimprove the thermal control of the assembly of buffer resistors R₁, R₂,R₃, and R₄ and a current regulating unit CR, the assembly of bufferresistors R₁, R₂, R₃, and R₄ and a current regulating unit CR areinsulated from the ambient temperature of the atmosphere in which thebattery pack 200 of FIG. 4 resides. The current regulating unit CR isshown as having a rectangular cross-section, and its four sidesaccommodate up to four buffer resistors. In embodiments having threeresistors R₁, R₂, and R₃, as shown in series-parallel battery packsdescribed above, the fourth resistor R₄ is not used. More generally, thecurrent regulating unit CR is not necessarily rectangular, and(depending on the number of batteries in each series) any number ofbuffer resistors can be attached to its side walls. In case each seriesin the pack 200 has less than four buffer resistors, some of the sitesfor buffer resistors can be vacant. The assembly of buffer resistors R₁,R₂, R₃, and R₄ and a current regulating unit CR has the buffer resistorsR₁, R₂, R₃, and R₄ disposed on the electrical insulation EI. Theelectrical insulation EI with the buffer resistors R₁, R₂, R₃, and R₄are secured to the current regulating unit CR to thermally couple thecurrent regulating unit CR to the buffer resistors R₁, R₂, R₃, and R₄.This entire construction is there encased in a thermal insulation thatisolates the assembly of buffer resistors R₁, R₂, R₃, and R₄ and acurrent regulating unit CR from the ambient. The structure of thethermal insulation TI and the thermal mass of the buffer resistors R₁,R₂, R₃, and R₄ and the current regulating unit CR are designed to thedesired activating temperature (temperature at which the bufferresistance increases sharply) and latency (from the rise of inrushcurrent to increase in buffer resistance). While not shown, any of theheat sink structures above described may be included in the assembly ofbuffer resistors R₁, R₂, R₃, and R₄ and a current regulating unit CR andbe in keeping with the principles of this invention.

While this invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

1. A current buffer assembly for limiting inrush current within abattery pack comprising a matrix of series-parallel connected batterycells, the current buffer assembly comprising: a plurality of currentlimiting devices wherein each of the plurality of current limitingdevices has a first terminal connected to a junction of two adjacentbattery cells on one column of the matrix of the series-parallelconnected batteries and a second terminal connected such that all thesecond terminals of the plurality of current limiting devices associatedwith one row of the series-parallel connected batteries are commonlyconnected; and a plurality of current limiting tracking means, whereineach of the plurality of current limiting tracking means are coupled tothe current limiting devices connected to the junctions of the twoadjacent battery cells on one column such that the current limitingprocess of each of the plurality of current limiting devices on the onecolumn limits an inrush current from parallel neighbor battery cells ofthe matrix of series-parallel connected battery cells from overchargingthe battery cells on the column having the shorted cell andover-discharging the other battery cells within the matrix ofseries-parallel connected battery cells when one battery cell on thecolumn shorts.
 2. The current buffer assembly of claim 1 wherein thecurrent limiting devices are buffer resistors.
 3. The current bufferassembly of claim 2 wherein the buffer resistors are positivetemperature coefficient buffer resistors.
 4. The current buffer assemblyof claim 3 wherein the positive temperature coefficient buffer resistorsare thermally coupled.
 5. The current buffer assembly of claim 4 whereinthe current limiting tracking means is a heat sink that thermallycouples the buffer resistors together such that the buffer resistorsincrease in resistance with a temperature increase caused by currentflowing through the buffer resistor associated with the shorted batterycell.
 6. The current buffer assembly of claim 5 wherein the heat sink iscopper, aluminum, tin, lead or other heat conducting material.
 7. Thecurrent buffer assembly of claim 1 further comprising a plurality ofcurrent regulating elements wherein each current regulating element ofthe plurality of current regulating elements is in series with thebattery cells of one column of the matrix of series-parallel connectedbattery cells for preventing a large inrush current to the column of thebattery cells on which the current regulating element is disposed. 8.The current buffer assembly of claim 7 wherein each of the currentregulating elements have a positive temperature coefficient to limit aninrush current when a battery cell on the column on which the currentregulating resistor is disposed is shorted.
 9. The current bufferassembly of claim 8 wherein each of the current regulating elements is avariable resistor, switch, relay, contact, a fuse, a current regulatorcircuit, or an electronic switch.
 10. The current buffer assembly ofclaim 7 wherein each of the current regulating elements are coupled tothe current limiting tracking means such that the current limitingprocess of each of the plurality of current limiting devices on the onecolumn limits an inrush current from parallel neighbor battery cells ofthe matrix of series-parallel connected battery cells from overchargingthe battery cells on the column having the shorted cell andover-discharging the other battery cells within the matrix ofseries-parallel connected battery cells when one battery cell on thecolumn shorts.
 11. The current buffer assembly of claim 10 wherein thevariable resistors are positive temperature coefficient bufferresistors.
 12. The current buffer assembly of claim 11 wherein thepositive temperature coefficient buffer resistors are thermally coupled.13. The current buffer assembly of claim 12 wherein the current limitingtracking means is a heat sink that thermally couples the bufferresistors together such that the buffer resistors increase in resistancewith a temperature increase caused by current flowing through the bufferresistor associated with the shorted battery cell.
 14. The currentbuffer assembly of claim 1 further comprising a thermal insulation meansencasing the plurality of plurality of current limiting devices and theplurality of current limiting tracking means to isolate the plurality ofcurrent limiting devices and the plurality of current limiting trackingmeans from the ambient temperature.
 15. A battery pack comprising: amatrix of rows and columns of series-parallel connected battery cells;an inrush current limiting circuit for limiting inrush current withinthe battery pack resulting from shorting of at least one of theseries-parallel connected battery cells, comprising: a plurality ofcurrent limiting devices wherein each of the plurality of currentlimiting devices has a first terminal connected to a junction of twoadjacent battery cells on one column of the matrix of theseries-parallel connected batteries and a second terminal connected suchthat all the second terminals of the plurality of current limitingdevices are commonly connected; and a plurality of current limitingtracking means, wherein each of the plurality of current limitingtracking means are coupled to the current limiting devices connected tothe junctions of the two adjacent battery cells on one column such thatthe current limiting process of each of the plurality of currentlimiting devices on the one column limits an inrush current fromparallel neighbor battery cells of the matrix of series-parallelconnected battery cells from overcharging the battery cells on thecolumn having the shorted cell and over-discharging the parallelneighbor battery cells within the matrix of series-parallel connectedbattery cells when one battery cell on the column shorts.
 16. Thebattery pack of claim 15 wherein the current limiting devices are bufferresistors.
 17. The battery pack of claim 16 wherein the buffer resistorsare positive temperature coefficient buffer resistors.
 18. The batterypack of claim 17 wherein the positive temperature coefficient bufferresistors are thermally coupled.
 19. The battery pack of claim 18wherein the current limiting tracking means is a heat sink thatthermally couples the buffer resistors together such that the bufferresistors increase in resistance with a temperature increase caused bycurrent flowing through the buffer resistor associated with the shortedbattery cell.
 20. The battery pack of claim 19 wherein the heat sink iscopper, aluminum, tin, lead or other heat conducting material.
 21. Thebattery pack of claim 15 wherein the inrush current limiting circuitfurther comprising a plurality of current regulating elements whereineach current regulating element of the plurality of current regulatingelements is in series with the battery cells of one column of the matrixof series-parallel connected battery cells for preventing a large inrushcurrent to the column of the battery cells on which the currentregulating element is disposed.
 22. The battery pack of claim 21 whereineach of the current regulating elements have a positive temperaturecoefficient to limit an inrush current when a battery cell on the columnon which the current regulating resistor is disposed is shorted.
 23. Thebattery pack of claim 22 wherein each of the current regulating elementsis a variable resistor, switch, relay, contact, a fuse, a currentregulator circuit, or an electronic switch.
 24. The battery pack ofclaim 21 wherein each of the current regulating elements are coupled tothe current limiting tracking means such that the current limitingprocess of each of the plurality of current limiting devices on the onecolumn limits an inrush current from parallel neighbor battery cells ofthe matrix of series-parallel connected battery cells from overchargingthe battery cells on the column having the shorted cell andover-discharging the parallel neighbor battery cells within the matrixof series-parallel connected battery cells when one battery cell on thecolumn shorts.
 25. The battery pack of claim 15 wherein the variableresistors are positive temperature coefficient buffer resistors.
 26. Thebattery pack of claim 25 wherein the positive temperature coefficientbuffer resistors are thermally coupled.
 27. The battery pack of claim 26wherein the current limiting tracking means is a heat sink thatthermally couples the buffer resistors together such that the bufferresistors increase in resistance with a temperature increase caused bycurrent flowing through the buffer resistor associated with the shortedbattery cell.
 28. The battery pack of claim 15 wherein the currentbuffer assembly further comprises a thermal insulation means encasingthe plurality of plurality of current limiting devices and the pluralityof current limiting tracking means to isolate the plurality of currentlimiting devices and the plurality of current limiting tracking meansfrom the ambient temperature.
 29. A method for limiting inrush currentwithin a battery pack comprising a matrix of series-parallel connectedbattery cells, the method for limiting inrush current comprising:providing a plurality of current limiting devices; connecting each ofthe plurality of current limiting devices to a first terminal connectedto a junction of two adjacent battery cells on one column of the matrixof the series-parallel connected batteries; commonly connecting a secondterminal of the plurality of current limiting devices associated withone row of the series-parallel connected batteries; and providing aplurality of current limiting tracking means; coupling each of theplurality of current limiting tracking means to the current limitingdevices connected to the junctions of the two adjacent cells on onecolumn such that the current limiting process of each of the pluralityof current limiting devices on the one column limits an inrush currentfrom parallel neighbor battery cells of the matrix of series-parallelconnected battery cells from overcharging the battery cells on thecolumn having the shorted cell and over-discharging the parallelneighbor battery cells within the matrix of series-parallel connectedbattery cells when one battery cell on the column shorts.
 30. The methodfor limiting inrush current of claim 29 wherein the current limitingdevices are buffer resistors.
 31. The method for limiting inrush currentof claim 30 wherein the buffer resistors are positive temperaturecoefficient buffer resistors.
 32. The method for limiting inrush currentof claim 31 wherein coupling each of the plurality of current limitingtracking means comprises thermally coupling the positive temperaturecoefficient buffer resistors.
 33. The method for limiting inrush currentof claim 32 wherein the current limiting tracking means is a heat sinkthat thermally couples the buffer resistors together such that thebuffer resistors increase in resistance with a temperature increasecaused by current flowing through the buffer resistor associated withthe shorted battery cell.
 34. The method for limiting inrush current ofclaim 33 wherein the heat sink is copper, aluminum, tin, lead or otherheat conducting material.
 35. The method for limiting inrush current ofclaim 29 further comprising: providing a plurality of current regulatingelements; arranging each current regulating element of the plurality ofcurrent regulating elements in series with the battery cells of onecolumn of the matrix of series-parallel connected battery cells forpreventing a large inrush current to the column of the battery cells onwhich the current regulating element is disposed.
 36. The method forlimiting inrush current of claim 35 wherein each of the currentregulating elements have a positive temperature coefficient to limit aninrush current when a battery cell on the column on which the currentregulating resistor is disposed is shorted.
 37. The method for limitinginrush current of claim 36 wherein each of the current regulatingelements is a variable resistor, switch, relay, contact, a fuse, acurrent regulator circuit, or an electronic switch.
 38. The method forlimiting inrush current of claim 35 wherein coupling each of the currentregulating elements to the current limiting tracking means causes thecurrent limiting process of each of the plurality of current limitingdevices on the one column to limit the inrush current from parallelneighbor battery cells of the matrix of series-parallel connectedbattery cells from overcharging the battery cells on the column havingthe shorted cell and over-discharging the parallel neighbor batterycells within the matrix of series-parallel connected battery cells whenone battery cell on the column shorts.
 39. The method for limitinginrush current of claim 38 wherein the variable resistors are positivetemperature coefficient buffer resistors.
 40. The method for limitinginrush current of claim 39 wherein the positive temperature coefficientbuffer resistors are thermally coupled.
 41. The method for limitinginrush current of claim 40 wherein the current limiting tracking meansis a heat sink that thermally couples the buffer resistors together suchthat the buffer resistors increase in resistance with a temperatureincrease caused by current flowing through the buffer resistorassociated with the shorted battery cell.
 42. The method for limitinginrush current of claim 41 wherein the heat sink is copper, aluminum,tin, lead or other heat conducting material.